SCANNING APPARATUS
A scanning apparatus operable in the microwave, mm-wave, sub mm-wave (TeraHerz) and infrared ranges comprises a primary drum (10) mounted for rotation about a central axis A of the primary drum being hollow and of rectangular polygonal form to provide a number of sides or facets (12, 14) each adapted to transmit such radiation, from a field of view, which is plane polarised in a first direction at 45° with respect to the rotary axis of the drum and to reflect radiation which is plane polarised in an orthogonal direction. Thus, radiation passing into the drum though whichever said side of the drum is currently facing the field of view and passing towards the diametrically opposite side will be plane polarised with a polarisation direction such as to be reflected back by that diametrically opposite side towards the rotary axis of the drum. Each polygon side of the drum is configured so as to act, when reflecting radiation striking that side from within the drum, as a concave mirror, to focus the radiation towards a receiver assembly which includes a radiation detector for such radiation. In another embodiment scanning apparatus operable in the microwave, sub mm-wave, mm-wave and infrared ranges may comprise a reflective disc or mirror (50′, 52′) mounted for rotation relative in a support (74, 76) is itself mounted for rotation with respect to a second support (86) about a second axis inclined with respect to the first axis.
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THE PRESENT INVENTION relates to a scanning apparatus operable in the infrared, sub mm-wave (TeraHerz), mm-wave or microwave ranges of electromagnetic radiation. It is an object of the present invention to provide an improved scanning apparatus operable with radiation of the wavelengths indicated, having a large effective aperture and which is able to repeatedly scan, at a high rate, a two-dimensional (e.g. altitude and azimuth) field of view, and which yet can be constructed at reasonably low expense.
In infrared imaging systems, use is frequently made of flapping mirrors and rotary polygons with reflective surfaces to scan the scene. In these infra red systems the pupil in the scanner is typically 10 mm in diameter. In mm-wave or microwave systems however the apertures to be scanned are frequently 1 m or larger in diameter and the use of large flapping mirrors at high scan rates (e.g. such as to provide ten field scans or “frames” per second or more) is not practical in these systems. It is known that tilted rotary discs may be used to scan large apertures but these produce a conical scan pattern or a linear scan with a large amount of pupil wander.
In accordance with a first aspect of the present invention, there is provided scanning apparatus operable in the microwave, mm-wave sub mm-wave (TeraHerz) and infrared ranges and comprising a support structure, a primary drum which is mounted in said support structure for rotation relative to the support structure about a central axis of the primary drum, said primary drum being hollow and internally of regular polygonal form to provide a number of sides or facets, (ideally, but not necessarily, an even number of sides or facets), each adapted to transmit such radiation which is plane polarised in a first direction at 45° with respect to the rotary axis of the drum and to reflect radiation which is plane polarised in a direction at 45° to the rotary axis of the drum and perpendicular to the said first polarisation direction, such radiation emanating from a field of view of the apparatus, being a field of view which is fixed with respect to said supporting structure, (as opposed to rotating with the primary drum), the arrangement being such that radiation passing into the drum through whichever said side of the drum is currently facing said field of view and passing towards the diametrically opposite side will be plane polarised with a polarisation direction such as to be reflected back by said diametrically opposite side towards the rotary axis of the drum, each said polygon side being configured so as to act, when reflecting such radiation striking that side from within the drum, as a concave mirror, to focus the radiation towards a receiver assembly which includes a radiation detector for such radiation.
In accordance with a second aspect of the invention there is provided scanning apparatus operable in the microwave, mm-wave sub mm-wave (TeraHerz) and infrared ranges and comprising a support structure, a first reflective disc or mirror which is mounted in said support structure for rotation relative to the support structure about a first axis, a second reflective disc or mirror which is mounted in said support structure for rotation relative to the support structure about a second axis, the arrangement being such that radiation from a scene being scanned can reach a reflective surface of the first disc or mirror to be reflected thereby onto a reflective surface of the second disc or mirror, to be reflected by the latter, in turn, onto a further part of the apparatus incorporating a receiver or receivers for such radiation, and wherein said reflective surface of the first disc or mirror has an axis of rotational symmetry, (or a normal where said surface is planar), tilted at a small angle relative to said first axis and wherein said reflective surface of the second disc or mirror has an axis of rotational symmetry (or a normal where said surface is planar) tilted at a small angle relative to said second axis, and driving means for said discs or mirrors arranged to drive these in respective opposite senses.
It is an object of the invention in yet another of its aspects to provide apparatus which can effectively simulate the action of a flapping mirror, without the problems associated with rapid changes of momentum which place practical limitations on the aperture size and rate of scan (rate of flap) of a flapping mirror.
In accordance with this further aspect of the invention there is provided scanning apparatus' operable in the microwave, mm-wave sub mm-wave (TeraHerz) and infrared-ranges and comprising a first support structure and a reflective disc or mirror which is mounted in said first support structure for rotation relative to the first support structure about a first axis and wherein the reflective surface of the first disc or mirror has an axis of rotational symmetry, (or a normal where said surface is planar), tilted at an angle relative to said first axis and in which said first support structure is itself mounted for rotation with respect to a second support structure about a second axis inclined with respect to said first axis at the same angle as that at which said axis of rotational symmetry or normal is tilted relative to said first axis, the apparatus including means for rotating said reflective disc or mirror on or in said first support structure about said first axis at a first rate relative to said second structure and means for rotating said first support structure, relative to said second support structure about said second axis at the same rotational rate as said first rate but in the opposite rotational sense from that in which said reflective disc or mirror is rotated, whereby said reflective disc or mirror can effect a back and forth linear scan in a field of view.
It will be understood that apparatus as set out above can be combined with further means for effecting an orthogonal scan at a different rate in a field of view to produce a two-dimensional raster scan of the field of view. Such further means may comprise a further linear scan apparatus as set out above or may comprise some other known means for producing a linear scan, for example a simple flapping mirror may be used to effect a field scan at a relatively low scan rate whilst scanning apparatus as set out in the preceding paragraph above, comprising a rotating and precessing reflective disc or mirror, effects a line scan at a substantially higher rate.
The present invention makes it possible to manufacture a system for providing multiple linear scans, in a compact arrangement that is suitable for use in high-speed mm-wave and microwave applications.
Embodiments of the invention are described below with reference to the accompanying schematic drawings in which:—
Referring to
As noted above, the sides of the drum, or at least the notional surfaces on which lie the wires of the wire grid polarisers, are not precisely planar, but are slightly concave on their sides facing towards the axis A, and are configured so as to form concave mirrors, so that, for example, as illustrated in
Thus, referring to
Referring to
It will be appreciated that radiation passing from outside the primary drum 10 from the scene being scanned, substantially normal to the primary drum side wall which is for the time being the entry side for such radiation, can be regarded as a beam of radiation from the scene scanned and which sweeps around the axis A as the drum 10 rotates. This beam, after reflection at the rotating secondary reflector/reflective member 16, is substantially stationary, allowing it to be collected effectively by a stationary radiation detector (not shown), which may be mounted within the drum 10. Thus, as the drum 10 rotates, one of its faces at a time is used to scan the scene. At the limit of the scan the radiation being collected by the radiation detector from the scene scanned passes from one face of the secondary reflective member 16 to the next and the next face of the rotating drum 10 becomes that through which the radiation reaching the radiation detector from the scene-scanned passes and thus the next scan begins.
The location of the stationary radiation detector or detector array is in general a matter to be determined by considerations of mechanical convenience, requirements for compactness, etc. However it is possible to take advantage of the polarisation of the radiation after reflection by the reflective member 16 to minimise radiation losses between reflective member 16 and the radiation detector array. For example, it may be convenient to locate the radiation detector or detector array outside the drum 10 and to reflect the radiation from the reflective member 16 through the side walls of the drum 10 to the radiation detector or detector array. In this case, it is generally necessary to, ensure that direction of polarisation of the radiation after reflection from reflective member 16 towards the radiation detector or detector array is rotated through 90 degrees (with respect to its direction of polarisation before striking the secondary reflective member), before the radiation reaches the side walls of drum 10 in order to allow the radiation to pass through such side walls. This may be done by placing a Faraday rotator or a quarter wave plate within the drum in front of reflective member 16 and between reflective member 16 and the radiation detector or detector array. Alternatively, as illustrated in
In the arrangement shown in
In another variant, illustrated in
Assuming the central axis A to be vertical, the features of the arrangements thus far described only provide a horizontal scan of the field of view. If, as is generally required, a two-dimensional field of view is required, a raster type scan in which the horizontal scan provides a line scan may be provided by any of the expedients described for this purpose in WO03/012524. As an example, a line scan may be provided by providing a vertical array of radiation detectors, so that the number of raster lines would equal the number of detectors in the array. Alternatively, the side walls of drum 10, instead of being strictly vertical, i.e. parallel with the axis A, (or rather instead of having their principal axes extending strictly radially with respect to the axis A), may be variously inclined slightly to the axis A so that successive faces of the drum 10 would cause a different horizontal scan line in the field of view to be focused on a single receiving element. By combining these possibilities it is possible to obtain a scanning raster in which the number of scan lines is equal to the product of the number of receivers in a vertical receiver array by the number of faces of the drum 10. Thus, if the drum 10 has six faces, as illustrated, the number of horizontal scan lines in the scanning raster will be six times the number of receivers in the array.
As a further possibility, in arrangements corresponding to those illustrated in
In a further variant, the secondary reflective member 16 could be configured as a prism having roof reflectors and the orthogonal or field scan could be achieved by displacing this prism along its axis of rotation. Either the whole prism could be displaced as a function of time or else this prism could be fixed and individual reflectors displaced with respect to their neighbouring reflectors as a function of time.
In the arrangement of
In the arrangement shown in
On reflection at the second rotating mirror 52, this radiation passes back through the second quarter wave plate or Faraday rotator 58 and is reflected at the inclined polariser 54 as illustrated by the ray diagram in
When the radiation is thus reflected for the second time by the plane polariser 54, it leaves the linear scanner arrangement shown (leaving towards the right in
In other embodiments, any one or both of the rotating mirrors may be curved. For example, the first rotating mirror 50 may be slightly curved to correct for spherical aberrations and may additionally be, concave to provide a converging effect on radiation reflected towards the second rotating mirror, whereby the size of the second rotating mirror may be reduced. It is not possible to reflect from a powered mirror at a significant off-axis angle without introducing serious aberrations. However, the use of the polarising beam splitter 54 inclined at 45° with respect to the rotary axis of discs 50, 52, between the two rotating discs, makes it possible to achieve a near-normal incidence and reflection of radiation at the first rotating mirror 50. The second mirror 53, in this arrangement, may, as shown in
In the scanning arrangement illustrated in
In this optical arrangement, the first rotating mirror is reflected from twice and its inclination must be half the effective inclination of the second rotating mirror. An advantage of this latter configuration is that pupil wander in a direction parallel to the direction of scan is effectively removed. The scanning mechanisms illustrated in
Referring again to
In yet another embodiment, the first and second rotating tilted mirrors perform both the high speed line scan pattern and the orthogonal scan (which completes the frame scan). In this case, each mirror rotates about two axes. Thus, referring to
The actual angles of inclination β and precession α depend on the desired fields of view and curvatures of each rotating mirror. For example, if the first mirror were substantially plane, then a tilt of 2.5° of this mirror and an effective tilt of 5° of the second mirror would achieve a horizontal line scan of plus or minus 20°. Similarly an angle of precession of 2.5° on the first mirror and an effective angle of precession of 5° on the second mirror could achieve a vertical frame scan also of plus or minus 20°.
The actual angles of tilt and precession on the second mirror are the effective values multiplied by (1−x/R) where x is the spacing between the upper and lower mirrors and where R is the radius of curvature of the second (lower) mirror 52 in
The frame rate and number of scan lines in the scan pattern depends on the actual speeds of the high-speed rotation and the lower speed precession. There are two scan lines per rotation and two frames per precession. So to achieve a frame rate of 10 Hz the precession speed is 300 rpm. Also with 100 scan lines per frame and 10 parallel receiver channels the speed of rotation is 3000 rpm. In this situation the actual number of useable scan lines is less than 100 since the scan lines overlap towards the top and bottom of the frame.
The high speed rotation and lower speed precession of the arrangement described above may be achieved, for each mirror using the mechanical arrangement shown in
In
There has been described above the addition of a precession to the rotational axis of the mirror discs of the rotating disc scanner illustrated in
In addition, the provision for precession of the rotational axis may also be applied to rotating polygon scanners such as that described with reference to
A further apparatus embodying the invention may have the same form as indicated schematically in
In use of the scanning mechanism described in the preceding paragraph to produce a two-dimensional image of scene, provision is made for bringing radiation reflected from the mirror to a focus in a focal plane or surface, e.g. by making the rotating mirror a concave mirror, or by providing some other focusing means, whereby the radiation reflected from the mirror is focused in the focal plane. In one embodiment, a linear array of detectors is placed at the focal plane, the direction in which the line of detectors in this detector array extends being again perpendicular to the direction of line scan. Alternatively, the radiation leaving this scanning mechanism may pass to a separate orthogonal scanner. The orthogonal (frame) scan may, for example, be achieved by a flapping mirror flapping at a rate significantly lower than the line scan rate. Alternatively, of course, a further rotating mirror with its rotary axis precessing at the same rate as, but in the opposite sense from, that further rotating mirror may be used to simulate the effect of the flapping mirror flapping at an appropriate frame scan rate. Likewise, a scanning mechanism as described with reference to any of
Claims
1. Scanning apparatus operable in the microwave, mm-wave sub mm wave (TeraHerz) and infrared ranges and comprising a support structure, a first reflective disc or mirror which is mounted in said support structure for rotation relative to the support structure about a first axis, a second reflective disc or mirror which is mounted in said support structure for rotation relative to the support structure about a second axis, the arrangement being such that radiation from a scene being scanned can reach a reflective surface of the first disc or mirror to be reflected thereby onto a reflective surface of the second disc or mirror; to be reflected by the latter, in turn, onto a further part of the apparatus incorporating a receiver or receivers for such radiation, and wherein said reflective surface of the first disc or mirror has an axis of rotational symmetry, (or a normal where said surface is planar), tilted at a small angle relative to said first axis and wherein said reflective surface of the second disc or mirror has an axis of rotational symmetry (or a normal where said surface is planar) tilted at a small angle relative to said second axis, and driving means for said discs or mirrors arranged to drive these in respective opposite senses.
2. Scanning apparatus as claimed in claim 1, wherein said first and second reflective discs or mirrors are both concave mirrors, arranged with their concave sides facing one another and wherein a wire grid polariser is located between the mirrors inclined at an angle with respect to the two mirrors so as to receive radiation, from a scene being scanned, arriving transversely with respect to said first and second axes and to reflect a plane polarised component of such radiation towards said first mirror, and wherein a quarter wave plate, Faraday rotator or equivalent device is located between said first mirror and said wire grid polariser, whereby the radiation passing to said first mirror and reflected thereby towards said second mirror has its polarisation direction shifted through 90 degrees in passing twice through said quarter wave plate, Faraday rotator or equivalent device, and can thus pass through said wire grid polariser to said second mirror to be focused by said second mirror onto a radiation detector or receiver.
3. Apparatus according to claim 2 wherein a further Faraday rotator or quarter wave plate or equivalent device is located between said wire grid polariser and said second mirror, whereby radiation passing through said second mirror and reflected thereby towards said first mirror has its polarisation shifted through 90° in passing twice through said second quarter wave plate, Faraday rotator or equivalent device and is reflected by said wire grid polariser, in a direction away from the scene being scanned, towards a radiation detector or receiver.
4. Apparatus according to claim 2 wherein said second mirror is arranged to direct said radiation to said radiation detector or receiver indirectly, by directing said radiation again through said quarter wave plate, Faraday rotator or equivalent device to said first mirror, to be reflected again, in turn, by said first mirror.
5. Apparatus according to claim 4 arranged so that after the second reflection by said first mirror and the subsequent passage through the first quarter wave plate, Faraday rotator or equivalent device, the radiation is reflected again by said wire grid polariser, towards said radiation detector or receiver.
6. Scanning apparatus operable in the microwave, mm-wave, sub mm-wave (TeraHerz) and infrared ranges and comprising a first support structure and a reflective disc or mirror which is mounted in said first support structure for rotation relative to the first support structure about a first axis and wherein the reflective surface of the first disc or mirror has an axis of rotational symmetry, (or a normal where said surface is planar), tilted at an angle relative to said first axis and in which said first support structure is itself mounted for rotation with respect to a second support structure about a second axis inclined with respect to said first axis at the same angle as that at which said axis of rotational symmetry or normal is tilted relative to said first axis, the apparatus including means for rotating said reflective disc or mirror on or in said first support structure about said first axis at a first rate relative to said second support structure and means for rotating said first support structure, relative to said second support structure about said second axis at the same rotation rate as said first rate but in the opposite rotational sense from that in which said reflective disc or mirror is rotated, whereby said reflective disc or mirror can effect a back and forth linear scan in a field of view.
7. Apparatus according to claim 6 in combination with further means for effecting an orthogonal scan at a different rate in a field of view to produce a two-dimensional raster scan of the field of view.
8. A scanning apparatus operable in the microwave, mm-wave, sub mm wave (TeraHerz) and infrared ranges and comprising a support structure, a primary drum which is mounted in said support structure for rotation relative to the support structure about a central axis of the primary drum, said primary drum being hollow and of rectangular polygonal form to provide a number of sides or facets each adapted to transmit such radiation which is plane polarised in a first direction at 45° with respect to the rotary axis of the drum and to reflect radiation which is plane polarised in a direction at 45° to the rotary axis of the drum and perpendicular to the said first polarisation direction, such radiation emanating from a field of view of the apparatus, being a field of view which is fixed with respect to said supporting structure, (as opposed to rotating with the primary drum), the arrangement being such that radiation passing into the drum through whichever said side of the drum is currently facing said field of view and passing towards the diametrically opposite side will be plane polarised with a polarization direction such as to be reflected back by said diametrically opposite side towards the rotary axis of the drum, each said polygon side being configured so as to act, when reflecting such radiation striking that side from within the drum, as a concave mirror, to focus the radiation towards a receiver assembly which includes a radiation detector for such radiation.
9. Apparatus according to claim 8 in which said radiation detector is stationary with respect to said support structure and said receiver assembly includes means whereby the radiation reflected from such diametrically opposite, side of the rotating drum reaches said radiation detector as a substantially stationary cone.
10. Apparatus according to claim 8 in which said receiver assembly includes a radiation reflective member mounted within the primary drum for rotation, in said support structure, about an axis coincidental with or parallel with said central axis of the primary drum, the apparatus including means for rotating said radiation reflective member at one half the speed of the primary drum, and in the same rotational sense, said radiation reflector having a plurality of radiation reflective facets and being such that, in section in a plane perpendicular to its rotary axis, said reflective facets define a regular polygon with twice as many sides as the primary drum, said receiver assembly further including means for receiving radiation reflected from said radiation reflective member, and for directing radiation so-received to said radiation detector.
11. Apparatus according to claim 10 wherein said means for receiving radiation reflected from said radiation reflective member is located outside the primary drum and wherein said radiation reflective member is arranged to reflect such radiation, to said means for receiving, through the sides of the primary drum, means being provided within the drum and interposed between said radiation reflective member and said means for receiving for rotating the polarisation direction of such radiation through 90° to pass through said sides of the primary drum.
12. Apparatus according to claim 10 wherein said radiation reflective member comprises a plurality of pairs of reflective facets, one facet of each pair being disposed further along said axis of the reflective member than the other, the number of such pairs being twice the number of facets of the primary drum, and wherein the facets of each pair are so arranged that radiation directed onto one of said facets after reflection from a said side of the primary drum will be reflected onto the other facet of the pair to be reflected thereby through the sides of the primary drum, to said means for receiving radiation.
13. Apparatus according to claim 12, wherein the two facets of each said pair are perpendicular to one another.
14. Apparatus according to claim 13 wherein the two facets of each said pair are inclined at opposite 45° angles to the rotary axis of said radiation reflective member.
15. Apparatus according to claim 10 wherein said means for receiving radiation comprises an element, herein referred to as a transreflector, mounted within the primary drum and arranged not to obstruct plane polarised radiation reflected from a said facet of the primary drum towards said radiation reflective member but to reflect directly or indirectly to said radiation sensing means, radiation reflected onto said transreflector by said radiation reflective member.
Type: Application
Filed: Feb 25, 2005
Publication Date: Feb 4, 2010
Patent Grant number: 8259378
Applicant: FARRAN TECHNOLOGY LIMITED (COUNTY CORK)
Inventor: Alan Harold Lettington (Berkshire)
Application Number: 10/594,351
International Classification: G02B 26/10 (20060101); G02B 26/12 (20060101);